Horizontal spacing in 2.0 compared to 1.3

I've been noticing differences in how 2.0 lays out scores compared to 1.3, and I think it's time we look into this further.

This began when I encounter submitted #24268: Barline to note distance too small for 1.3 scores. I noticed that when 2.0 read in 1.3 scores, the first note of each measure was too close to the barline. This doesn't happen in 1.3, and it doesn't happen in 2.0 for scores created from scratch. It turns out 2.0 is deliberately forcing 1.3 scores into this state.

When I tried fixing this, I realized that adding back the missing space was affecting the layout - number of bars per system - more often that I'd have liked. By having 2.0 force 1.3 scores to crowd the barlines, this was masking differences elsewhere in the layout algorithms - although to be sure, there were still some cases were 1.3 scores were taking more space in 2.0 than in 1.3.

I am not sure where this difference comes from - probably multiple sources. But I can observe that you can see it in a completely empty score. Create a score with one staff from scratch in 1.3 and in 2.0. Delete the staff name in 1.3. Make sure both scores have the same page size, margins, and scaling. Make sure you are using all default Style settings. With letter size paper, 1.3 will fit 22 empty measures on a system, 2.0 can only fit 18. I thought this might be because of differences in the settings in Style / General / Measure, but even after making 2.0 mimic 1.3 in all corresponding settings, I still get only 18 measures per system.

Seems this is worth further discussion and investigation.